For example, formation of a multilayer coating film on an object to be coated, such as an automobile body, is performed basically by sequentially forming an intermediate coating film, a base coating film and a clear coating film on a steel sheet which is an object to be coated. Heretofore, these coating films have been each formed by applying a coating composition with a composition adjusted according to the function of the coating film, and baking and curing each of the applied coating compositions.
Unless an underlying layer is completely formed and smoothed when a plurality of coating compositions are applied one after another, adjacent coating film layers interfere with each other, so that irregularities of the underlying layer are reflected on the upper layer, leading to deterioration of the appearance of the multilayer coating film.
However, for improving work efficiency, and particularly realizing energy saving which has been highly required in recent years, a method for forming a multilayer coating film, which includes applying a plurality of coating compositions one after another in an uncured state, and then curing these coating compositions at a time, For example, a three-coating and one-baking method including the step of baking and curing an intermediate coating composition, a base coating composition and a clear coating composition at a time has been progressively employed.
In addition, in a coating method such as a three-coating and one-baking method in which a coating composition is continuously applied in an uncured state, the coating film appearance is markedly deteriorated if sagging occurs during application of an intermediate coating composition that is applied first, and therefore a technique for preventing sagging during application of an intermediate coating composition has been required.
Japanese Patent Laid-open Publication No. 2009-262001 discloses a method for forming a multilayer coating film having an excellent coating film appearance by controlling not only the viscosity of an intermediate coating composition but also the ratio of the hydrophilic property and the hydrophobic property of a resin to be used in the intermediate coating composition in a three-coating and one-baking method, so that sagging during application of the intermediate coating composition is effectively prevented.